sensors Article Geosynchronous Satellite GF-4 Observations of Chlorophyll-a Distribution Details in the Bohai Sea, China Lina Cai 1, Juan Bu 1, Danling Tang 2,*, Minrui Zhou 1, Ru Yao 1 and Shuyi Huang 1 1 Marine Science and Technology College, Zhejiang Ocean University, Zhoushan 316004, China; [email protected] (L.C.); [email protected] (J.B.); [email protected] (M.Z.); [email protected] (R.Y.); [email protected] (S.H.) 2 Southern Marine Science and Engineering Guangdong Laboratory Guangdong Key Laboratory of Ocean Remote Sensing (LORS), South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China * Correspondence: [email protected] Received: 5 August 2020; Accepted: 21 September 2020; Published: 24 September 2020 Abstract: We analyzed the distribution of chlorophyll-a (Chla) in the Bohai Sea area based on data from the geosynchronous orbit optical satellite Gaofen-4 (GF-4), which was launched in 2015, carrying a panchromatic multispectral sensor (PMS). This is the first time the geosynchronous orbit optical satellite GF-4 remote-sensing data has been used in China to detect the Chla change details in the Bohai Sea. A new GF-4 retrieved model was established based on the relationship between in situ Chla value and the reflectance combination of 2 and 4 bands, with the R2 of 0.9685 and the total average relative error of 37.42%. Twenty PMS images obtained from 2017 to 2019 were applied to analyze Chla in Bohai sea. The results show that: (1) the new built Chla inversion model PMS-1 for the GF-4 PMS sensor can extract Chla distribution details in the Bohai Sea well. The high Chla content in the Bohai Sea is mainly located in coastal areas, such as the top of Laizhou Bay, Bohai Bay and Liaodong Bay, with the value being around 13 µg/L. The concentration of Chla in the Bohai Strait and northern Yellow Sea is relatively low with the value being around 5 µg/L. (2). Taking full advantage of the continuous observation of geostationary orbit satellite, GF-4 with a high-resolution sensor PMS of 50 m can effectively detect short-term change (changes within 10 min) in Chla concentration. The changes mainly appear at the southwest and northeast costal area as well as in the center of Bohai Sea with the change value of around 3 µg/L. (3) The change of Chla concentration in the Bohai sea is related to the environmental factors such as seawater temperature, salinity, illumination and nutrient salts, as well as the dynamic factors such as wind, flow field and tidal current. Keywords: GF-4; chlorophyll-a; Bohai Sea 1. Introduction The Bohai Sea, located in north eastern China, is a semi-closed shallow sea, into which a large amount of sewage is discharged along the Gulf Coast. Environmental pollution of the Bohai Sea is becoming increasingly serious. The eutrophication of the water body is significantly intensified, resulting in frequent occurrence of red tides [1]. As an important part of marine environmental factors and water quality parameters, chlorophyll-a (Chla) has great significance for the marine ecological environment [2]. The detection of Chla concentration based on remote-sensing technology aims to calculate the Chla concentration using off-water reflectance [3] after atmospheric correction [4] and geometric correction of remote-sensing Sensors 2020, 20, 5471; doi:10.3390/s20195471 www.mdpi.com/journal/sensors Sensors 2020, 20, 5471 2 of 17 satellite image data. In prior studies, different inversion models [5] are developed to retrieve Chla concentration according to the off-water emissivity. For the typical type II water body in the Bohai Sea, it is an urgent task to monitor the water color and quality change details of the sea [6]. Chla concentration can help us monitor the water quality and understand the characteristics of the coastal waters, as well as provide scientific basis for the management, and comprehensive utilization of the Bohai Sea [7–9]. It will be significant to find a more suitable Bohai Sea algorithm of Chla concentration inversion for the new sensor. The inversion algorithms for detecting Chla based on remote-sensing technology can be divided into two categories: empirical formula methods and model-based analytical methods [10]. At the beginning of 1990, Gitelson calculated the Chla concentration based on the ratio of the red band and the infrared band [11]. Meanwhile, neural network and semi-analytical algorithms, as well as bio-optical models were also used to invert Chla concentration [12]. Shu Xiaozhou adopted Gitelson’s method, corrected by using the phycoerythrin absorbance in 624 nm spectra, to improve the calculation accuracy of Chla [13]. In China, many prior studies analyzed the correlation [14,15] between the normalized difference and the in situ Chla concentration, and found that the two show a significant negative correlation. The Chla in type II waters retrieved from the incineration algorithm shows that the average absolute error of the results is 1.081 [16]. Using satellite remote-sensing data, the detection of Chla concentration of type II water is greatly affected by suspended sediment, colored dissolved organic matter (CDOM), etc. [17]. A multi-factor algorithm was established previously by researchers for ocean water color hyperspectral information. The quantitative relationship between the common contributions of water color factors such as Chla, suspended sediment, and CDOM and the emissivity of water was developed based on high-resolution and hyperspectral remote-sensing fusion data [18]. At present, there are many empirical and semi-empirical algorithms for marine Chla inversion, and most of them use satellite data, such as Moderate-resolution Imaging Spectroradiometer (MODIS) data and Operational Land Imager (OLI) data [19–22], from the near-polar solar synchronous orbit. In recent years, due to the influence of manmade and natural factors, the environment of China’s coastal waters has undergone great changes, and the nutrient salt structure of some sea areas has changed, resulting in increasingly frequent red tide disasters [23]. Prior studies found that, in general, the annual variation of Chla concentration in the far coastal waters of Bohai bay is relatively low. Furthermore, the Chla concentration variation is characterized by a large range of continuous changes, and the seasonal variation in the near shore waters is obvious due to the influence of current conditions and precipitation [2]. Most important of all, the effect of tidal wave motion on Chla concentration distribution cannot be ignored. Tidal wave movement is the most important marine dynamic process in the Bohai Sea and plays an important role in controlling the marine environment [24]. The Gaofen-4 (GF-4) satellite is a geosynchronous orbit optical satellite with an orbital height of 36,000 km and a high resolution of 50 m, which has the advantages of realizing real-time cryptic observation of the marine environment [25,26]. GF-4 can observe disaster events such as algal blooms and red tides through pointing control to provide fast and reliable optical remote-sensing data [27]. The GF-4 satellite is the Earth observation satellite with the highest geostationary orbit resolution in the world. The electronic maritime surveillance satellite constellation basically meets the needs of ship surveillance in terms of coverage and time resolution [28]. It not only has continuous surveillance capabilities and large-scale coverage capability, but also has a geostationary orbit (GEO) high-resolution optical detection with fast response capability and higher positioning accuracy (better than 200 m) [29]. It can realize continuous monitoring of key targets, and make up for the problems of low time resolution and small coverage of traditional reconnaissance satellites. Meanwhile, it will also launch a series of new application areas, including the ability to move across the surface of the Earth based on video shooting methods, target detection and monitoring [30], long-term evolution monitoring of various natural elements of the Earth, etc. Furthermore, it has a quick response capability, and the delivery time from user’s application to geospatial information can be reduced to a few minutes. GEO high-resolution optical imaging satellite is a new generation of optical remote-sensing satellite [31]. Sensors 2020, 20, 5471 3 of 17 Sensors 2020, 20, x FOR PEER REVIEW 3 of 18 “movingIts outstanding target featuressurveillance” can be and summarized the other in twois “rea words,l-time one surveillance”. is “moving target It raises surveillance” the satellite and reconnaissancethe other is “real-time function surveillance”. to the surveillance It raises thefunction, satellite which reconnaissance can realize function the dramatic to the surveillance change of satellitefunction, application. which can realizeIn addition, the dramatic GEO high-score change of satellitesatellites application. can enhance In the addition, value GEOof information high-score applicationssatellites can based enhance on thenew value product of information types [29]. applications However, there based is on no new suitable product empirical types [ 29model]. However, based onthere GF-4, is no a high-resolution suitable empirical satellite model in based geosynchronous on GF-4, a high-resolution orbit, for Chla satellite inversion in geosynchronousin the coastal waters orbit, suchfor Chla as Bohai inversion Sea. inTherefore, the coastal it is waters urgent such for asus Bohaito develop Sea. Therefore, Chla inversion it is urgent algorithms for us tofor develop GF-4. Chla inversionIn this algorithms paper, the forconcentration GF-4. distribution and change details of Chla is analyzed based on the panchromaticIn this paper, multispectral the concentration sensor (PMS) distribution data of the and newly change launched details high-resolution of Chla is analyzed GF-4 based satellite on inthe synchronous panchromatic orbit. multispectral Based on sensor GF-4 satellite (PMS) data PMS of data the newly and field launched measurement high-resolution data, an GF-4 empirical satellite algorithmin synchronous model orbit.of Chla Based inversion on GF-4 is established.